Conventional composite structures are typically made by laminating many layers of "pre-preg", which contain reinforcing fibers surrounded by a matrix impregnated with a resinous binder substance, thereby building up the shape desired for the final structure. One common type of composite, consisting of an epoxy matrix reinforced by many small diameter graphite fibers, is called graphite reinforced epoxy (GRE). Another uses a specialized binder substance called PMR-15 as its matrix.
A complex cycle of curing the binder substance at elevated temperatures and pressures is required to transform the layered structure into a rigid assembly. the binder must first liquify and flow to penetrate all of the fiber layers, then solidify into a uniform matrix around the reinforcing fibers. Various combinations of pressure and temperature must be used during the cure cycle to compensate for changes in the viscosity of the binder. Small deviations from the desired cycle can result in a variety of defects in the finished part, such as delamination of adjacent layers or entrapment within the matrix of small bubbles of gas generated by the curing reaction. These defects result in an unfavorable reduction in the mechanical performance of the finished assembly. An in-situ nondestructive technique for carefully monitoring the progress of the cure cycle is therefore very desirable.
Attempts to overcome some of the difficulties encountered with the complex curing cycles called for with these materials include the use of thermocouples to monitor temperatures during the cure cycle, but this method provides only indirect information about the progress of the transformation from flexibility to rigidity. Capacitive sensors that infer rigidity from changes in the dielectric polarizability of the binder have met with only limited success in current manufacturing applications.
In U.S. Pat. No. 4,921,415 to Thomas, III et al., assigned to the same assignee as the present invention, there is disclosed apparatus for monitoring the curing of fiber reinforced composite plastic that is cured at temperatures on the order of 350.degree. C., based on the utilization of an ultrasonic transducer assembly.
The embedded NMR sensor approach as taught in the present invention admirable overcomes many of the problems of cure cycle monitoring and control experienced with such prior art techniques. Due to its in-situ cure state sensing capabilities and its compatibility with existing NMR spectrometer techniques, it provides a significant improvement to the field of cure monitoring the control of composite structures.